Invariant Natural Killer T (iNKT) cells are a lipid-specific lymphocyte subset restricted to the non-polymorphic antigen presenting molecule CD1d. We recently demonstrated that iNKT cells can enhance B cell humoral responses. However, the specific contribution of iNKT help for B cell responses during immune defense against infectious pathogens and vaccine antigens remains largely undefined. The overall objective of this proposal is to understand the mechanism of how iNKT cells help B cells to promote both immunity and the response to polysaccharide-based vaccines. The working model for this proposal is that the nature and duration of iNKT cell cytokine production and reactivity dictate the humoral memory outcome in response to glycolipid antigens, infectious pathogens, and polysaccharide-based vaccines. Our rationale for this hypothesis is based on our previously published studies dissecting iNKT and B cell cooperation in response to model glycolipid antigens. Streptococcus pneumoniae is the ideal infection model for investigating these interactions because both B cells and iNKT cells are critical for survival from S. pneumoniae infection, human antibody titers stimulated by polysaccharide-based S. pneumoniae vaccines correlate with NKT cell numbers, and, despite the introduction of effective protein-conjugate polysaccharide vaccines, S. pneumoniae remains a major public health threat responsible for the deaths of nearly half a million children each year. We will achieve our objective in three steps: In Aim1 we will utilize mixed bone marrow chimeras, FACS analysis, confocal imaging, and serum antibody ELISA/ELISPOTs to identify the duration of cell-type specific cytokine contributions to iNKT cell supported humoral memory. In Aim2, we will apply the same techniques to dissect the iNKT cell effector functions responsible for driving antigen-specific humoral B cell outcomes during infection. Finally, in Aim3 we will couple ELISA and ELISPOT approaches to studies with S.pneumoniae polysaccharide-based vaccines and a new nanoparticle formulation to define the potential for an-iNKT ligand to serve as a vaccine adjuvant. Importantly, we expect that by learning the mechanisms of cognate and non- cognate iNKT cell help for B cells during immune responses against glycolipid antigens, bacterial infection, and polysaccharide-based vaccines we will be better able to exploit these cells for improved immune defense and vaccine development against many different human pathogens. These findings are expected to have an important positive impact by informing future development of an entirely new, iNKT-directed, vaccine strategy which would be complementary for existing vaccine approaches and, given the non-polymorphic nature of CD1d, would be universally effective in all people.